Research work has been ongoing in recent years on various types of display devices, including liquid crystal display devices, electroluminescence (EL) display devices, plasma display panel display devices, and they have been commercialized as display devices for the information processing apparatus, such as computers and televisions. The TFT type liquid crystal display devices (also called TFT LCDs from here on) are particularly widespread, their market size is expected to grow even further, and accordingly, there is a need for further improvements in image quality. For example, display devices with high contrast ratio under all types of environments are needed. While the descriptions below are for the examples of TFT LCD, the present invention is not limited to the TFT LCD and can be applied to a simple matrix LCD or a plasma addressed LCD, or the like, and can in general be applied on all types of LCDs, in which the liquid crystal is held between a pair of substrates, on each of which electrodes are formed, respectively, and a display is made with a voltage applied between the respective electrodes. Furthermore, it is also applicable to so-called self light emitting display devices, including the organic EL display devices, inorganic EL display devices, and PDP display devices.
So-called VA mode liquid crystal display devices have been proposed (see, for example, Patent Document 1), in which the liquid crystal having a negative dielectric anisotropy is aligned vertically between substrates facing one another, in order to meet the requirement for a high contrast ratio. As disclosed in Patent Document 1, when no voltage is applied, the liquid crystal cells in a VA mode liquid crystal display device have almost no birefringence or optical rotation, because the liquid crystal molecules are aligned in an essentially vertical direction with respect to the substrate surface, and the light transmits through the liquid crystal cells with almost no changes in its polarization state. Accordingly, it is possible to realize an essentially perfect black color display when no voltage is being applied, with a pair of polarizers (linear polarizers) placed above and below the liquid crystal cells in such a way that their absorption axes are essentially orthogonal to each other (also called crossed nicols polarizers). When a voltage is applied, the liquid crystal molecules become tilted and essentially parallel to the substrate, exhibiting a large birefringence, and a white color is displayed. Accordingly, such a VA mode liquid crystal display device is able to easily realize a very high contrast ratio.
Nevertheless, there has been room for further improvement, even with the VA mode liquid crystal display device having the construction described above, because the contrast ratio would decrease when viewed under outside light, such as sunlight, or under lighting equipment. It is caused by an apparent rise in the screen brightness under the effect of the reflected outside light off of the display showing the black color, which should be dark. The main causes of the reflection of the outside light are thought to be (1) a reflection off of the topmost surface of the liquid crystal display device on the side which is being observed; and (2) reflection internal (inside the liquid crystal cells) to the liquid crystal display device. The latter includes the reflection off of the metal wiring on the TFT substrate, the reflection off of the transparent electrodes (typically ITO), and the reflection off of the black matrix, formed for color separation of the color filter layers and for blocking light to the TFT device, or the like.
A technology for forming an anti-reflective film, having an anti-reflective effect on the observer side of the polarizing film, which is formed on the topmost surface of the liquid crystal display device—i.e., the observer side of the liquid crystal cells, for reducing the reflection of outside light is widely known and has been commercialized to address the reflection off of the topmost surface of the liquid crystal display device on the observer side, as described above in (1). In general, a polarizing film used for a TFT LCD includes a protective film (typically a TAC film) on its topmost surface on the observer side, and its refractive index is approximately 1.5. For this reason, it has a reflectance of approximately 4% with respect to the incident light incoming from air, when there is no anti-reflective film. On the other hand, it is possible to reduce this to 2% or less by forming the aforementioned anti-reflective film. Nevertheless, a reduction in reflection inside the liquid crystal cells cannot be achieved even when this technology is used.
Accordingly, methods of reducing the reflection inside the liquid crystal cells, as described in (2) above, are being explored, and various proposals have been made (see, for example, Patent Documents 2 through 6). For example, a method of using a low reflectance metal, such as chromium oxide, for the black matrix has been disclosed. In addition, development work on non-metallic, resin based black matrix materials has also been underway and commercialized. Furthermore, Patent Document 2 discloses a method in which an anti-reflective transparent layer is formed between the black matrix and the transparent substrate (typically glass). Nevertheless, according to further studies undertaken by the inventors, it was found that these conventional reflection reduction technologies cannot be applied as remedies on the reflection off of the metallic wiring lines on the TFT substrate or the reflection off of the ITO transparent electrodes, and anti-reflective effects are obtained on limited portions.
Patent Document 3 discloses a method in which the optical transmittance of the visible light in the polarizing plate on the observer side is made lower than those on the light source side. It is possible to reduce reflectance on all portions inside the liquid crystal cells with this method. Nevertheless, according to the investigations undertaken by the present inventors, it was found that the anti-reflective effect obtained with this method is small.
Furthermore, Patent Document 4 discloses a method in which a circularly polarizing plate is placed on the side of the transparent electrode in an organic electroluminescence device. It is possible to reduce reflectance on all portions inside the liquid crystal cells with this method also. However, according to the investigation undertaken by the present inventors, it was found that the anti-reflective effect with respect to the outside incident light from the normal direction of the display device is large, but the anti-reflective effect with respect to the outside incident light from oblique directions, which are different from the direction normal to the display device, is inadequate with this method using the conventional circularly polarizing plate.
On the other hand, Patent Document 5 discloses a method in which the anti-reflective effect on the circularly polarizing plate is obtained across a wide view angle with the lamination of a λ/2 plate with the NZ coefficient of 0.1 to 0.4 and a λ/4 plate with the NZ of 0.3 to 0.7 in such a way that their optical axes intersect. Furthermore, Patent Document 6 discloses a method in which the anti-reflective effect is obtained on the circularly polarizing plate across a wide view angle with the lamination of a λ/2 plate with the NZ coefficient of 0.6 to 0.9 and a λ/4 plate with NZ of 0.3 to 0.7, in such a way that their optical axes intersect.